Unit overhaul 1/6: Add unit-conversion utilities and FitTarget

[henrikjacobsenfys]

Additive foundation for the unit-system overhaul, split out of the larger branch so it can be reviewed and merged on its own.

• utils.py: add convert_value_unit, convert_parameter_unit, convert_units_with_rollback, energy_to_scipp, verify_Q_index and _assert_valid_unit helpers, plus the kb constant and CANONICAL_Q_UNIT. No behavior change to existing functions.
• fit_target.py: new FitTarget dataclass describing one fittable model prediction bound to a dataset key.
• test_utils.py: tests for the new helper functions.

All unit tests pass.

---

Stack — splitting the monolithic "Unit system overhaul" (#217) into a reviewable series. Merge bottom-up:

1. Add unit-conversion utilities and FitTarget → develop (Unit overhaul 1/6: Add unit-conversion utilities and FitTarget #221) (this PR)
2. Centralize Q-index validation via verify_Q_index → split/01-unit-utils (Unit overhaul 2/6: Centralize Q-index validation via verify_Q_index #222)
3. Store Q as a scipp Variable in 1/angstrom → split/02a-verify-q-index (Unit overhaul 3/6: Store Q as a scipp Variable in 1/angstrom #223)
4. Separate model unit into x_unit/y_unit + update tutorials → split/02b-q-variable (Unit overhaul 4/6: Separate unit into x_unit/y_unit and update tutorials #224)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.92%. Comparing base (6de7065) to head (e79b9d7).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #221      +/-   ##
===========================================
+ Coverage    97.89%   97.92%   +0.03%     
===========================================
  Files           52       53       +1     
  Lines         3653     3707      +54     
  Branches       650      660      +10     
===========================================
+ Hits          3576     3630      +54     
  Misses          45       45              
  Partials        32       32              

Flag	Coverage Δ
unittests	97.92% <100.00%> (+0.03%)	⬆️

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Files with missing lines	Coverage Δ
src/easydynamics/utils/fit_target.py	100.00% <100.00%> (ø)
src/easydynamics/utils/utils.py	98.86% <100.00%> (+1.08%)	⬆️

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